Instead, you need to translate between ViewModels and domain entities and vice versa. The domain entities do not belong directly to the ViewModel. The ViewModel is a data model exclusively for presentation layer needs. This reason is what the ViewModel is for. Therefore, entities should not be bound to client views, because at the UI level some data might still not be validated. The point here is that the domain entity is contained within the domain model layer and should not be propagated to other areas that it does not belong to, like to the presentation layer.Īdditionally, you need to have always-valid entities (see the Designing validations in the domain model layer section) controlled by aggregate roots (root entities). Then part of that information, or an aggregation of information including additional data from other entities, can be sent to the client UI through a REST Web API. Different layers (like the domain model layer versus the presentation layer, etc.) might have different types, which mandate translations between those types.įor example, an entity could be loaded from the database. They exist to help developers manage the complexity in the code. The layers are a logical artifact, and are not related to the deployment of the service. Most enterprise applications with significant business and technical complexity are defined by multiple layers. If a microservice must rely on another service to directly service a request, it is not truly autonomous. If two microservices need to collaborate a lot with each other, they should probably be the same microservice.Īnother way to look at this aspect is autonomy. It is similar to the Inappropriate Intimacy code smell when implementing classes. Cohesion is key within a single bounded context. You should balance them by decomposing the system into as many small microservices as you can until you see communication boundaries growing quickly with each additional attempt to separate a new Bounded Context. Second, you want to avoid chatty communications between microservices. First, you want to initially create the smallest possible microservices, although that should not be the main driver you should create a boundary around things that need cohesion. Keep the microservice context boundaries relatively smallĭetermining where to place boundaries between Bounded Contexts balances two competing goals. DDD is about boundaries and so are microservices. The components within those boundaries end up being your microservices, although in some cases a BC or business microservices can be composed of several physical services. And that is explicit in the form of a microservice. You build and refine a domain model that is contained within a boundary that defines your context. For the domain model for each Bounded Context, you identify and define the entities, value objects, and aggregates that model your domain. DDD patterns help you understand the complexity in the domain. Where to draw the boundaries is the key task when designing and defining a microservice. Simpler responsibilities, like a CRUD service, can be managed with simpler approaches. In addition, DDD approaches should be applied only if you are implementing complex microservices with significant business rules. But the important part is not the patterns themselves, but organizing the code so it is aligned to the business problems, and using the same business terms (ubiquitous language). Sometimes these DDD technical rules and patterns are perceived as obstacles that have a steep learning curve for implementing DDD approaches. This section introduces the design and implementation of those internal patterns. It also suggests many technical concepts and patterns, like domain entities with rich models (no anemic-domain model), value objects, aggregates, and aggregate root (or root entity) rules to support the internal implementation. It describes independent problem areas as Bounded Contexts (each Bounded Context correlates to a microservice), and emphasizes a common language to talk about these problems. In the context of building applications, DDD talks about problems as domains. Domain-driven design (DDD) advocates modeling based on the reality of business as relevant to your use cases.
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